Hydrothermal treatment of sludge within the pulp and paper industry can have a wide range of possible reactor solid loads for different sludge types. The objectives of this work were to determine the effect of reactor temperature and solid load on the properties of sludge hydrochar and to simulate hydrothermal treatment of sludge within a pulp and paper mill. Laboratory experiments were first performed within reactor temperature and solid load ranges of 180-260 degrees C and 10-50%, respectively, and the effects of sludge treatment were determined by comparing parallel mill-scale simulations. Based on the results, both reactor temperature and solid load had a statistically significant effect on the solid, ash, carbon and energy yields of sludge hydrochar. Increasing solid load minimized carbon dissolution to the liquid phase and increased the solid and energy yield of the attained hydrochar. According to mill-scale simulations in a 9 m(3) batch reactor, treating primary sludge alone produced an energy surplus of 22-36 GJ through char incineration, decreasing to 12-22 GJ and 3.4-9.1 GJ for mixed and secondary sludge, respectively. Mixing primary and secondary sludge reduced the overall energy surplus by 2-8% compared with treating the sludge streams separately in two reactors. (C) 2016 Elsevier Ltd. All rights reserved.